1. Field of the Invention
The present invention relates to a power factor correction (PFC) circuit. More specifically, the present invention relates to a PFC circuit for controlling a phase of an input current to be identical with a phase of an input voltage by adjusting a duty ratio of a switch using the input current detected on the ground potential.
2. Description of the Related Art
FIG. 1 shows a conventional PFC circuit. A filter 1 and a bridge diode 2 rectify AC power from an AC 14 source to supply an input voltage Vs smoothened by a capacitor C1. An input current i.sub.L is supplied to a primary side of an inductor L, which is switched according to the ON and OFF states of the switch 3, which is a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Accordingly, an output voltage Vo is supplied to a load. A diode D1 and a capacitor C2 rectify the output voltage Vo so as to supply the rectified voltage Vout to an adjustable load Ro.
The circuit also includes a controller 40. A reference voltage Vref is supplied to an error amplifier 4 through a noninverting input terminal. A voltage, produced by dividing Vout with two resistors R1 and R2, is supplied to the error amplifier 4 through an inverting input terminal. The error amplifier 4 amplifies a difference voltage by a predetermined ratio, and outputs the amplified voltage Veo to an subtractor 5. The subtractor 5 subtracts the reference voltage Vref from the voltage Veo, and outputs a voltage V2.
A resistive divider made by resistors R3 and R4 produces a voltage V1. A multiplier 6 multiplies the voltage V2 and a voltage V1, and outputs a multiplied voltage Vmo.
A comparator 7 compares the voltage Vmo with a voltage of a resistor Rs which is coupled between the source of the switching MOSFET 3 and the ground. A latch circuit 8 logically latches signals provided from the comparator 7 and a current detector 9, and outputs the latched signal to a logical NOR gate 10.
The current detector 9 detects a current flowing through the inductor L in order to control the switch 3. The logical NOR gate 10 executes a logical NOR operation on the signals provided from the latch circuit 8 and the current detector 9, so as to control the switch 3. Therefore, when the input voltage Vs is increased (or reduced) and the voltage V1 which detects the amount of fluctuation that the input voltage Vs is increased (or reduced), the multiplied voltage Vmo is increased (or reduced), and the turn on duty ratio of the switching MOSFET 3 is altered, thereby, the output voltage Vo is increased (or reduced).
However, since the resistor Rs to detect the input current flowing through the inductor is coupled between the source of the switching MOSFET and the ground, when the switching MOSFET is turned on, it exhibits poor characteristics, such as switching noise. Additionally, all three of the input current, input voltage, and the output voltage need to be detected in order to compensate for the power factor. This complicates the circuit and increases its cost.